1. Field of the Invention
This invention relates to a surface active agent comprising as its major component a secondary amidoamino acid or a specific salt thereof and, more particularly, a surface active agent which has good compatibility with various additives such as water-soluble polymers or the like, and yet exhibits excellent low-temperature stability as an aqueous solution. The invention also relates to a process for preparing such a surface active agent.
2. Description of the Background
In recent years, there have been increasing demands for household products with high safety and low irritation. The trend is applicable to surface active agents as well, and general preference is directed to surface active agents which are low or only moderately irritating to human skin or eyes. For this reason, imidazoline type surface active agents are being more widely accepted as a base material for shampoos and various kinds of other detergents. imidazoline type surface active agents are manufactured by amphoterizing an alkyl imidazoline, i.e., by reacting an alkyl imidazoline with monochloroacetic acid or its alkali metal salt. The imidazoline type surface active agents thus prepared have long been considered as possessing an imidazoline structure. However, recent studies have revealed the fact that the compounds have a structure derived by ring-opening of imidazoline which is represented, for example, by the following formula (II) or (III): ##STR2## in which M represents an alkali metal and R has the same meaning as defined above.
Today, these surface active agents are still called imidazoline type surface active agents in the art because of this historical background, but in this specification they are described as "amidoamino acid type surface active agents" in order to correctly reflect their structures.
Accordingly, these amidoamino acid type surface active agents can be manufactured by reacting an alkyl imidazoline or an amidoamine which is the hydrolyzate of an alkyl imidazoline with monochloroacetic acid or its alkali metal salt. However, since this reaction is carried out in the presence of an alkali, NaOH for example, in order to neutralize acid to increase the reaction yield, the resulting amidoamino acid surface active agents inevitably contain a great amount of an inorganic salt, NaCl for example, which is a byproduct of reaction.
These amidoamino acid type surface active agents containing a great amount of an inorganic salt have several drawbacks. That is, they (i) increase the viscosity of the final product, (ii) decrease the low temperature stability of a formulated product, and (iii) are difficult to formulate with other active ingredients, e.g., the compounded products may cause emulsion breaking. For these reasons, there have been various limitations imposed on formulating these amidoamino acid type surface active agents to products.
A number of studies have, therefore, been carried out for decreasing byproduct inorganic salts contained in these glycine type amidoamino acid surface active agents. For instance, a method to refine such surface active agents by means of solvent extraction is proposed in Japanese Patent Application Disclosure No. 75998/1984. This method, however, requires elimination and recovery of the extraction solvent which involves more cost. Also, the products obtained do not always satisfy the required compatibility with other additives and low-temperature stability.
In these situations, the present inventors have made earnest and extensive studies in order to resolve the above problems, and found that the drawbacks as mentioned above can be resolved by converting a counter ion of an amidoamino acid type surface active agent represented by formula (II) hereinbelow into hydrogen, ammonium ion or triethanolammonium ion by a specific method, and at the same time by decreasing the salt content to less than a certain level. These findings have led to the completion of this invention.